This work seeks to engineer, optimize, and validate new tools and strategies to control protein activity with light, which can be delivered with spatial and temporal resolution. In previous work, we pioneered use of the Arabidopsis thaliana blue light photoreceptor cryptochrome 2 (CRY2) as an optogenetic module. We developed a light-induced dimerization platform based on interaction with a partner protein, CIB1, which can be used to control activity and localization of target proteins. In recent work, we developed a new photoregulated module, CRY2olig, that induces protein oligomerization with light. The broad aims of this project are to further develop the CRY2/CIB and CRY2olig technologies allowing control of protein dimerization and oligomerization with light. Specifically, we aim to optimize CRY2/CIB dimerizers with improved and extended properties for cell biology applications (Aim 1). In Aim 2, we will apply these dimerizers to control activity of a set of important and versatile regulatory enzymes, allowing manipulation of DNA recombination, protein cleavage, and protein labeling with light. Importantly, we will carry out further optimization and validation of a promising light-activated Cre recombinase, activity of which we will test in vivo in rodent brain. In Aim 3, we will develop and validate methods to transiently and locally disrupt protein activity using CRY2 and CRY2olig. Such tools will enable researchers to quickly and precisely disrupt function, avoiding compensatory effects that can occur with genetic ablation or knockdown studies.